This invention relates to monolithic microwave integrated circuits (MMICs), and more particularly to auxiliary circuitry for such MMICs.
As is known in the art, MMICs are used in many applications. Typically these circuits are formed on a gallium arsenide semiconductor substrate because of the high frequency signals used in such circuits. One type of active device used in such circuit is a heterojunction bipolar transistor (HBT). Thus, for example, a MMIC amplifier used to amplify microwave signals, for example a microwave signal received from a cellular phone tower, may include a plurality of cascaded amplifier stages, each stage having an HBT.
As is also known in the art, dc biasing is required for proper operation of an amplifier. For silicon bipolar transistor amplifiers, one arrangement is to use a grounded emitter configuration with dc biasing being provided using a current source. The current source typically includes a current mirror to maintain a constant collector current in the presence of temperature, fabrication process and supply voltage variations. With an enhanced current mirror, a pair of bipolar transistors having grounded emitters is provided. The collector of a first one of the transistors may be connected to a voltage through a relatively high impedance. The collector of the first transistor is also connected to the base of an emitter follower bipolar transistor. The emitter follower transistor has its emitter connected to the base electrodes of the pair of transistors. The collector of the emitter follower transistor and the second one of the pair of transistors are connected to a common supply voltage. The current through the collector of the second transistor mirrors the current fed to the collector of the first transistor. When the current mirror is fabricated as an integrated circuit, such as on a common silicon semiconductor substrate, the voltage drop across the base-emitter junction of the second transistor will be around 0.7 volts. Thus, the voltage at the base of the emitter follower transistor will be around 1.4 volts. In applications requiring a relatively low supply voltage, say a nominal supply voltage of 3.1 volts, even with variations in the supply voltage, say a supply voltage of only 2.8 volts, there is enough head-room with the 1.4 volts at the base of the emitter follower transistor to enable proper operation of the current mirror. Temperature and process variations will alter the bipolar transistor""s Vbe voltages resulting in a shift in all currents. Proper operation is when the current variation caused by temperature, process and voltage variations is sufficiently small so as to maintain a relatively constant collector current and subsequent constant amplification.
However, in a microwave application where a gallium arsenide substrate is used, the transistors being typically HBT devices, the voltage drop across the emitter-base junction will be about 1.25 volts. Therefore, with such HBTs, the voltage at the base of the emitter follower transistor will be about 2.5 volts. In the low supply voltage application described above (i.e., with a supply voltage of 2.8 volts), there is insufficient headroom to enable proper operation of this HBT current mirror and large variations in operating current will result.
In accordance with the present invention, an amplifier is provided. The amplifier includes a first single crystal semiconductor substrate having formed thereon at least one input signal amplifying device, such device having a bipolar transistor. A second single crystal semiconductor substrate is provided. The second substrate is a material different from the material of the first substrate. A current mirror is included. The current mirror includes a plurality of electrically interconnected active devices, one portion of the devices being bipolar devices formed on the first substrate and another portion of the active devices comprising an insulated gate field effect transistor formed on the second substrate.
With such an arrangement, the amplifier may be used to amplify microwave signals; such amplification being provided by HBTs formed on the first material substrate. Further, the gain provided by the IGFET produces a much smaller voltage drop between the base electrode of the HBT amplifying device and the gate of such IGFET, resulting in sufficient headroom for the arrangement to operate with relatively low voltage supplies while minimizing the detrimental effects of both temperature, process and voltage variations.
In one embodiment, the first single crystal substrate is III-V material and the second single crystal substrate is silicon.
In one embodiment, the first single crystal substrate is gallium arsenide.
In one embodiment the bipolar devices are HBTs.
In one embodiment the insulated gate field effect transistor is a MOS device.
In accordance with another feature of the invention, the current mirror includes: a first single crystal semiconductor substrate; and, a second single crystal semiconductor substrate, such second substrate being a material different from the first substrate; a plurality of electrically interconnected active devices, one portion of the devices being bipolar devices formed on the first substrate and another portion of the active devices comprising an insulated gate field effect transistor formed on the second substrate.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.